As is known, numerous MEMS devices are available today. In particular, MEMS reflectors are known, which include mobile elements formed by mirrors.
In general, a MEMS reflector is designed to receive an optical beam and to vary the direction of propagation thereof, via a mirror. Typically, the direction of propagation of the optical beam is varied in a periodic or quasi-periodic way so as to carry out a scan of a portion of space with the reflected optical beam.
In greater detail, MEMS reflectors of a resonant type are moreover known. In general, a resonant MEMS reflector comprises an actuation system that causes oscillation of the respective mirror in a substantially periodic way about a resting position, the period of oscillation being as close as possible to the resonance frequency of the mirror in order to maximize the angular distance covered by the mirror during each oscillation, and hence maximize the size of the portion of space scanned.
Among resonant MEMS reflectors, biaxial MEMS reflectors are moreover known, where the mirror oscillates about two different axes, perpendicular to one another, with frequencies approximately equal to the respective resonance frequencies of the mirror with respect to the aforesaid axes.
In the context of the generation of images using resonant biaxial MEMS reflectors, it is known to adopt markedly different resonance frequencies for the two scanning axes. For example, resonant biaxial MEMS reflectors are known having their two resonance frequencies equal, for example, to 18 kHz and 600 Hz. Moreover, irrespective of the specific values of the resonance frequencies, when an image is formed using a resonant biaxial MEMS reflector, the latter directs the reflected optical beam in such a way that it follows a Lissajous trajectory. Consequently, the full image is obtained as set of interlaced complementary images.
This having been said, the use of resonant biaxial MEMS reflectors entails the generation of images affected by flicker. To overcome this drawback, the image-refresh rate may be increased up to values much higher than sixty frames per second. Since, according to another point of view, the flicker phenomenon can be interpreted as an imperfect coverage of each frame, the increase in the refresh rate renders this phenomenon less perceptible to the human eye.
In order to reduce the flicker phenomenon, the paper by Hofmann et al., “Wafer level vacuum packaged two-axis MEMS scanning mirror for pico projector application”, Proceedings of SPIE, Vol. 8977 89770A-11 (incorporated by reference), suggests adoption of a biaxial structure with high resonance frequencies, which differ by 60 Hz. In practice, the aforementioned paper proposes a resonant biaxial MEMS reflector with an actuation system of an electrostatic type, where both of the resonance frequencies are relatively high (one is 14.9 kHz and the other is 15.6 kHz), the difference between them being 700 Hz. This enables reduction of the refresh rate to values of less than sixty frames per second, without the flicker phenomenon excessively damaging the quality of the images. However, unfortunately there are not known solutions that enable precise control of the difference between the two resonance frequencies, even for particularly low values of this difference and in the case of operating bands that reach high frequencies (for example, between 20 kHz and 30 kHz). In this connection, it should be noted how in theory the adoption of high resonance frequencies close to one another enables, given the same refresh rate, a higher resolution to be obtained, as well as a better coverage of the images.
There is a need in the art to provide a MEMS device that will solve at least in part the drawbacks of the known art.